Probing the Magnetic Properties of Three Interconvertible Redox States of aSingle-Molecule Magnet with Magnetic Circular Dichroism Spectroscopy

Mathieu Gonidec,†,§ E. Stephen Davies,‡ Jonathan McMaster,*,‡ David B. Amabilino,*,† andJaume Veciana*,†,§

Institut de Ciencia de Materials de Barcelona (CSIC) and Networking Research Center on Bioengineering,Biomaterials and Nanomedicine (CIBER-BBN), Campus UniVersitari de Bellaterra,

E-08193 Bellaterra, Barcelona, Spain, and School of Chemistry, UniVersity of Nottingham,UniVersity Park, Nottingham NG7 2RD, U.K.

Received November 11, 2009; E-mail: vecianaj@icmab.es; amabilino@icmab.es; j.mcmaster@nottingham.ac.uk

Single-molecule magnets (SMMs), which are capable of exhibit-ing magnetic bistability1 and present unique quantum properties,2,3

have a range of potential applications in nanoscience includinginformation storage4 and molecular spintronics.5 The anionicdouble-decker phthalocyanine complexes of Tb, Dy, and Ho behaveas single-molecule magnets.6,7 While efforts have been made tostudy the magnetic behavior of their one- and two-electron oxidizedforms,8,9 no magnetic hysteresis data have been reported. Herein,we present the spectroelectrochemical characterization of theanionic, neutral, and cationic equivalents of a terbium double-deckercomplex (1, Figure 1) and report a study of the correspondinghysteresis of magnetization using magnetic circular dichroism(MCD) spectroscopy in frozen dilute solutions at low temperature(1.5 K). Our results show that low temperature MCD spectroscopyis a viable and powerful technique to study the magnetizationproperties of this interesting family of compounds.

MCD spectroscopy has been shown to be a powerful tool forthe optical detection of the magnetic behavior of Mn12 clusterSMMs.10-13 An advantage that MCD spectroscopy offers, whencompared to standard magnetometric measurements, is highsensitivity; this allows measurements of the magnetization of acollection of isolated molecules in dilute frozen solutions withdiamagnetic solvents, thus minimizing the effect of intermolecularmagnetic interactions and facilitating measurements at the single-molecule level.

We synthesized the neutral complex 1 following a previouslyreported procedure.14 The redox properties of 1 were determinedby cyclic voltammetry (Figure S1), and the oxidation and reductionprocesses at E1/2 ) 0.45 and 0.09 V vs SCE, respectively, werestudied further by UV/vis spectroelectrochemistry at 273 K. Thespectroscopic changes in the absorption spectra associated with theone-electron electrochemical interconversions of [Pc2Tb]+/0/- areshown in Figure S2. These demonstrate that [Pc2Tb]+/0/- speciesmay be electrogenerated reversibly and are stable on the time scaleof the experiment. The UV/vis spectra of [Pc2Tb]+/0/- (Figure 1)show the characteristic absorption bands of double-decker phtha-locyanine complexes, including Soret bands at 387, 371, and 358nm and Q bands at 705, 663, 624 nm, respectively.15 In the caseof [Pc2Tb]+/0, the characteristic π-radical band is observed at 495and 475 nm, respectively.15

The samples for MCD spectroscopy, as solutions in CH2Cl2 with0.8 M [NBu4][BF4], were frozen in liquid nitrogen, which affordedglasses of suitable quality for optical measurements. In situ absorptionspectra support the absence of aggregation of [Pc2Tb]+/0/- under the

conditions used (Figure S3-S5), an important feature of theseexperiments in which we probe the magnetic properties of acollection of molecules for which intermolecular magnetic interac-tions are minimized.

The rich MCD spectra of solutions of [Pc2Tb]+/0/- (Figure 2)contain prominent temperature and field dependent pseudo-A termsat 701, 660, and 624 nm, respectively, that correspond to the intenseQ-band features in the UV/vis absorption spectra (Figure 1). Inaddition, the fingerprint π-radical band of [Pc2Tb]0 and [Pc2Tb]+

show temperature-dependent MCD bands of opposite sign at 484and 510 nm, respectively.

† Institut de Ciencia de Materials de Barcelona (CSIC).§ CIBER-BBN.‡ University of Nottingham.

Figure 1. UV/vis absorption spectrum of solution of [Pc2Tb]0, in CH2Cl2

with 0.4 M [NBu4][BF4], and of solutions of [Pc2Tb]+/- generatedelectrochemically from [Pc2Tb]0. All spectra recorded at 273 K.

Figure 2. MCD spectra of solutions of [Pc2Tb]+/0/- at 1.5 K and 7 T inCH2Cl2 with 0.8 M [NBu4][BF4].

Published on Web 01/25/2010

10.1021/ja9095895 2010 American Chemical Society1756 9 J. AM. CHEM. SOC. 2010, 132, 1756–1757

The MCD intensity was monitored at 705, 664, and 631 nm for[Pc2Tb]+/0/-, respectively, over a B ) ( 2 T field range at a sweeprate of 1 T ·min-1 and plotted as magnetization hysteresis curves(Figure 3). The field dependent MCD intensity of solutions of[Pc2Tb]+/- recorded at 705 and 631 nm, respectively, (Figure 3,top) both present butterfly shaped hysteresis curves that are almostsaturated at B ) (2 T, with an abrupt drop at B < 0.09 T. Thesedata are similar to the hysteresis curves obtained by magnetometryon dilute solid solutions of [NBu4][Pc2Tb] in the isostructuraldiamagnetic [NBu4][Pc2Y] host,16 suggesting that MCD of frozensolutions of [Pc2Tb]+/0/- is a viable probe of the magnetic propertiesof isolated [Pc2Tb]+/0/- complexes. The principal differencesbetween the responses for [Pc2Tb]+ and [Pc2Tb]- include a largercoercive field (0.071 T compared to 0.023 T) and a stronger remnantsignal (0.198 as compared to 0.065 of the saturated signal intensity)for [Pc2Tb]+.

The sharp drop in magnetization at B < 0.09 T for the dilutesolid solution of [Pc2Tb]- arises from eight |Jz> |Iz> states whichare brought to resonance by the Zeeman splitting.17 This resonanceonly occurs at low field because of the small energy differencesbetween these states. The very similar behaviors observed in theMCD of frozen solutions of [Pc2Tb]+/-, especially the coincidenceof the change in magnetization around B ) ( 0.09 T, suggest thatthe source of their magnetic behavior is closely related. Thedifference in the width of their hysteresis curves above ap-proximately 0.2 T suggests that the |Jz> multiplet splitting is largerfor [Pc2Tb]+ than for [Pc2Tb]-. This is consistent with thetemperature dependence of the maximum in the imaginary com-ponent of the ac-susceptibility measurements previously reportedfor these oxidation states.9 The field dependent MCD of [Pc2Tb]0

recorded as a frozen solution at 664 nm exhibits different hysteresisbehavior as compared to those recorded for [Pc2Tb]+/-. Notably,the hysteresis is maintained over the whole range of magnetic field,between (1.5 T, and does not narrow around B ) 0 T. Conse-

quently, the coercive field is larger for [Pc2Tb]0 (0.16 T) than for[Pc2Tb]+/- in these frozen solutions. The derivatives of thehysteresis cycles of [Pc2Tb]+/0/- show features at ca. (0.4-0.5 Tthat may arise from quantum tunneling effects (Figures S6-S8),although the sharp decrease in the gradient at 0 T strongly indicatesthat in all three systems there is relatively little tunneling at zerofield. The relatively smooth profile of the hysteresis of [Pc2Tb]0

suggests that tunneling is not dominant in this complex whencompared to [Pc2Tb]+/-.

To conclude, the optical MCD technique used here for thecharacterization of bis(phthalocyaninato)lanthanide-based SMMsdoes not rely on the preparation of solid solutions and on theidentification of suitable isostructural diamagnetic hosts to minimizeintermolecular magnetic interactions. Furthermore, the MCD methodcan be used to probe the magnetic properties of a range of differentredox states of the same complex under essentially the same frozensolution conditions. While data from magnetic susceptibility studiessuggest that the cationic form, [Pc2Tb]+, is more attractive for SMMapplications,8,9 we show that, under the conditions of our experi-ment, the neutral complex, [Pc2Tb]0, has a greater coercive fieldthan the oxidized or reduced states.

Acknowledgment. This work was supported by the Marie CurieEST FuMaSSEC, EU NoE MAGMANet (515767-2), EMOCIONA(CTQ2006-06333/BQU) projects and by CIBER de Bioingenierıa,Biomateriales y Nanomedicina (CIBER-BBN, promoted by ISCIII),Spain.

Supporting Information Available: Cyclic voltammetry, spectro-electrochemistry, in situ absorption data of the frozen samples,derivatives of the hysteresis cycles. This material is available free ofcharge via the Internet at http://pubs.acs.org.

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JA9095895

Figure 3. Hysteresis curves of the normalized MCD intensity vs B recordedat 1.5 K and a sweep rate of 1 T ·min-1 for the Q-band of [Pc2Tb]+/- (top)and [Pc2Tb]0 (bottom). The symbols represent 1 of every 5 data points.

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